1. Field of the Invention
This invention pertains to the drawworks for oil drilling rigs and more specifically to a computer controlled system for controlling either or both the upward and the downward speed of the load bearing traveling block assembly, especially as it approaches one of its traveling limits.
2. Description of the Prior Art
It is well known in the oil drilling art (which includes drilling for gas as well as for oil) to utilize a drawworks in connection with the oil drilling rig or derrick to hold and to raise and lower, as desired, a drill string into and out of the associated well bore. Generally, the raising and lowering operation is accomplished by means of a traveling block having an appropriate hook or other similar assembly. The traveling block is secured in block-and-tackle fashion to a stationarily secured crown block or other limit fixture located at the top of the well derrick or rig. Although the load bearing assembly could take another form in a particular drilling rig embodiment, for purposes herein all such load bearing assemblies regardless of appearance are, for convenience, referred to as the "traveling block", which term also includes the hook or other attachment means, the associated equipment or other load associated therewith as it moves upwardly and downwardly.
The raising and lowering operation of the traveling block is controlled by means of a hoist cable, line or rope, one end of which is secured to the rig floor, thereby forming a so-called "dead" line. The other end of such line is secured to the drawworks proper, thereby forming the "fast" line. This assembly is generally operated and controlled by an operator sometimes also referred to as "the driller".
The drawworks generally includes a rotatable cylindrical drum upon which the fast line is wound utilizing a suitable prime mover, which includes a power transmission assembly. Thus, in association with the raising of the traveling block, the prime mover is controlled by an operator, usually referred to as the "driller", by way of a foot or hand throttle. Similarly, in connection with the lowering operation, the drawworks is supplied with one or more suitable brakes, also controlled by the driller, usually with hand controls. Generally, the primary brake, which typically is a friction brake, is supplemented with an auxiliary brake, often of the eddy current type or a magnetic brake, which can be employed independently or together to control the rate of lowering the traveling block.
As mentioned, the drawworks is usually fitted with a primary friction brake, which generally is either a band or a disk type. Also as mentioned, either or both the primary and the auxiliary or secondary brake can be used together or independently to control the speed of the traveling block. When the traveling block is being lowered, speed control is principally by way of the auxiliary brake and the final stopping of the traveling block is by way of the primary brake. At all times, the brakes are operated or controlled by the driller.
It may be apparent from the description so far that, inasmuch as a typical load borne by the traveling block can be 400 tons or even more, an operational error by the operator or driller or a failure in any of the systems controlling the speed or rate of upward or downward movement of the traveling block could be hazardous and even catastrophic, resulting in damage to equipment, personal injury and even loss of human life.
Attempts at automating the drilling operation of raising and/or lowering the traveling block have taken many forms so as to remove human judgment or the possibility that human error might be the reason for a resulting damaging failure. For example, a simple governor on the prime mover would ensure that the throttle speed could not exceed a predetermined limit so as to reduce the margin of error at slowing and stopping the travel block at its upper limit of travel to prevent the traveling block from ramming the crown block.
U.S. Pat. No. 4,434,971, Codrey, which issued Mar. 6, 1984, discloses a load overspeed control system for preventing brake burnout that would otherwise be caused by allowing a loaded traveling block to ascend or descend too fast as it approaches the top or bottom of its travel path and then suddenly administering the brake. A load signal is developed by a load sensor attached to the dead line. A position signal is produced by a position encoder attached to the drawworks' drive shaft and a velocity signal is produced by differentiating the position signal. A digital computer is pre-loaded with information pertaining to the maximum energy absorbing capability of the primary brake. When the traveling block comes within a predetermined distance to the crown block or derrick floor at an excessive amount of speed for the load, an emergency signal is produced to activate an emergency brake to prevent primary brake burnout or a crash.
As noted above, the position measurement in the Cordrey system is developed from sensing the rotation of the drive shaft of the rotating drum of the drawworks. As a cable or line winds and unwinds from a drum, the amount of cable for each rotation will vary because of changes in circumference of the reel. Further, cable stretch will vary as the load increases or decreases. In short, the position signal and the velocity signal developed from such sensing is indirect and often inexact, introducing possible errors in calculations that can defeat the procedure, thereby causing the often disastrous results mentioned above. It is further noted that the Cordrey system also does not coordinate both braking systems in gradual slowdown fashion, but only kicks in a second brake when an emergency signal is produced. Additionally, in slowing and stopping the upward movement of the traveling block, a brake is operated to oppose the drive force initiated by the throttle and there is no automated throttle control.
Therefore, it is a feature of the present invention to provide improved speed control for slowing down and stopping the traveling block of a drilling rig as it approaches an upper limit and/or a lower limit.
It is another feature of the present invention to provide improved speed control for slowing down and stopping the traveling block of a drilling rig utilizing direct and absolute position sensing for developing position and speed signals.
It is still another feature of the present invention to provide improved speed control for slowing down and stopping the traveling block of a drilling rig utilizing a process controller preprogrammed with instructions for slowing down and stopping the traveling block within a specified upper range and/or a specified lower range depending on the speed and load of the traveling block at predetermined locations before such upper range and/or before such lower range.